The Name.
It is used to avoid naming the name of God, Yahweh.
Deuteronomy 5:11 Exodus 20:7.
H' (Hashem) is Judaism's way of saying that the only name that really identifies its God is one that does not even name it, because they consider that nothing existing covers the reality of God. Other interpretations ascribe Hashem the meaning "I Am", that is what the divinity would have answered to Moses when he asked his name in the burning bush in Horeb.